The present invention relates to an apparatus for controlling energization of glow plugs used in starting internal combustion engines such as diesel engines.
For starting an internal combustion engine such as a diesel engine, the glow plugs are preheated by passing an electric current therethrough. It is preferable to preheat the glow plugs in as short a period of time as possible by speeding up the rate of increase of the temperature of the glow plugs. To meet such a demand, there has been proposed to use quick-heating glow plugs and two relays for the control of energization of the glow plugs, as disclosed in Japanese Laid-Open Utility Model Publication No. 56157383. According to the disclosed system, the glow plugs are quickly heated up to a predetermined temperature by a large current supplied from the first relay, and then the glow plugs are supplied with a small surrent from the second relay through a current limiting resistor to keep the temperature of the glow plugs at a desired level.
However, the current limiting resistor causes a wasteful consumption of electric power when the current is supplied from the second relay to the glow plugs. When the glow plugs are frequently used to provide a so-called "afterglow" after the engine has been started, or the glow plugs are energized for a long period of time, the current limiting resistor consumes as much electric power as the glow plugs do. This shortens the service life of the battery used, and is problematic from the standpoint of saving electric energy and natural resources.
Japanese Laid-Open Utility Model Publication No. 54-42827 discloses a glow plug control system free of the foregoing difficulty. This prior control system includes a circuit for controlling an electric current fed to glow plugs, the circuit having relays operable for selectively connecting at least two glow plugs in series with or parallel to each other. The known relay-controlled system is costly to construct since it requires at least four relay contacts to cause the direction of a current flowing through each the glow plugs to remain unchanged before and after the glow plug connection is varied.